The present invention relates to image display systems. In particular, the present invention relates to miniature image display system usable for camcorders, digital cameras, or helmet-mounted displays and other wearable applications.
In the field of miniature image display systems there are continuing challenges to design smaller, lighter, and more energy efficient systems. These challenges stem from the fact that a miniature image display system should preferably be small enough and light enough to for digital cameras or camcorders, or be wearable (mounted on a helmet or on eyeglasses). Further, these goals should be preferably achieved without sacrificing image quality, in particular, contrast ratio and brightness. Such systems may be used for wearable computer systems, gaming systems, viewfinder for camera and camcorders, distance interactions between people or between people and machines, virtual-reality system, and for many other applications.
Typically, desktop computer systems and workplace computing equipment utilize CRT (cathode ray tube) display screens to display images for a user. The CRT displays are heavy, bulky, and not easily miniaturized. For a laptop, a notebook, or a palm computer, flat-panel display is typically used. The flat-panel display may use LCD (liquid crystal displays) technology implemented as passive matrix or active matrix panel. The passive matrix LCD panel consists of a grid of horizontal and vertical wires. Each intersection of the grid constitutes a single pixel, and is controlled by a LCD element. The LCD element either lets light through or blocks the light. The active matrix panel uses a transistor to control each pixel, and is more expensive.
The flat-panel display typically requires external lighting to allow human eyes to see the images displayed on the display panel. This is because flat-panel displays do not generate their own light. For laptop, notebook, or palm computers, external lighting is typically positioned at the back of the flat-panel. The backlighting allows the user to see the images from the front of the flat-panel.
The flat-panels are also used for miniature image display systems because of their compactness and energy efficiency compared to the CRT displays. For miniature image display systems, reflective lighting, rather than the backlighting, is preferred. This is because using the reflective lighting technique, miniature image display systems can be designed having higher energy efficiency compared to the energy efficiency of image display systems designed using the backlighting techniques. The passive matrix displays rotate s-polarized light into p-polarized light when the display is switched on while acting as a normal reflective surface when switched off. Various configurations of miniature display systems using flat-panel display and reflective lighting technique can be found in U.S. Pat. No.5,808,800.
A typical example of a miniature display system 100 is illustrated in FIG. 1. Referring to FIG. 1, the light source 102 is typically one or more LED's (light-emitting diodes). To achieve uniform illumination to reflective type of display, the illumination must be both spatially and angularly uniform, with the angular extent given by the acceptance angle of the viewing optics. That is, preferably, the angle at which the light hits the special light modulator 104, or the display panel 104, is perpendicular to the plane of the display panel 104 and the filed angle of the viewing optics.
Typically, a collecting lens 106 is used to collect light from one or more light sources into a slightly convergent light beam to match the telecentricity of the viewing optics. And, an array of micro lenses or a diffuser 108 is used to provide diffusion. Because the light source 102, the collector 106, and the diffuser 108 are positioned on the sides of the display panel 104, the manufacturing of the system 100 is difficult and costly.
The light source 102 must be outside the field of view of a user so as not to block the image generated by the display. Therefore, a polarizing beam splitting cube ("PBS cube") 110 is often used is used to redirect the light. The PBS cube 110 includes a polarizing beam splitter (PBS) 112 which typically reflect s-polarized light while allowing p-polarized light to pass.
There are several problems associated with such design. Firstly, the bulk of the system 100 is difficult to reduce because distance between the first viewing optics and the display must be at least as great as the shortest dimension of the display. This is because the system 100 must allow sufficient space for the placement of the PBS cube 110.
Secondly, the bulk of the system 100 is difficult to reduce because the system 100 requires the use of the collecting lens 106 and the diffuser 108 for energy efficient operation. Generally, without the collecting lens 106 and the diffuser 108 much if not most of the light produced by the light source 102 would be wasted.
Thirdly, the bulk of the system 100 is difficult to reduce because the diffuser 108 must be at least as large as the display. This is because, in the illustrated prior art, the diffuser 108 is placed on the side of the collecting lens 106 which is the opposing side of the light source 102.
Finally, energy efficiency of the system 100 is low. There are several reasons for this. Reason one, only a portion 114 of the light from the light source 102 is captured by the collecting lens 106 and is directed toward the PBS cube 110. Some 116 of the light from the light source 102 is not captured by the collector 106 and is lost. This is because the light from the light source 102 is typically Lambertian. Lambertian light is light that is emitted in a radiation pattern in which the luminous intensity varies as the cosine of the off-axis angle, and is typically spread to about 120 degrees. In comparison, typical collection lens collects light for about 60 degrees.
Reason two, about 1/2 of the light 118 from the collector 106 and diffuser 108 is lost because the PBS 112 reflects only the s-polarized light. Accordingly, p-polarized light 120 is transmitted such that it will not each the display panel 104.
Reason three, after reflecting off the display panel 104, the light encounters the PBS 112 again. Again, only the p-polarized light 126 passes through the PBS 112 toward the optic lens 130 for viewing at an imaging area 132. All s-polarized light 128 is reflected by the PBS 112 and is lost.
Assuming that the collector 106 captures about 1/2 of the light produced by the light source 102, the system 100 of FIG. 1 allows only about 1/8 of the light produced by the light source 102 to eventually reach the optic lens 130. This is due to the combined losses at the collector 106 and at the PBS 112. This rough estimate does not take into account other losses. For example, energy is lost at the surface of the PBS cube 110 each time the light enters or leaves the PBS cube 110.
In sum, there exists continuing need for more compact, lightweight, and energy efficient display system that eliminates or minimizes these problems.